A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic assembly might be a lithographic apparatus, or might be a lithographic apparatus and additional equipment (e.g. a measuring tool, a radiation source, and the like). A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to impart a beam of radiation with a pattern in its cross-section, the pattern corresponding to a circuit pattern to be formed on an individual layer of the IC. This pattern can be imaged or transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions (which might be dies) that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an image of the entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In the semiconductor manufacturing industry there is increasing demand for ever-smaller features and increased density of features. The critical dimensions (CDs) are rapidly decreasing and are becoming very close to the theoretical resolution limit of state-of-the-art exposure tools such as steppers and scanners as described above. Conventional technologies aimed at enhancing resolution and minimizing printable CD include reducing the wavelength of the exposure radiation, increasing the numerical aperture (NA) of the projection system of the lithographic apparatus, and/or including features in a patterning device pattern smaller than the resolution limit of the exposure tool so that they will not print on the substrate, but so that they will produce diffraction effects which can improve contrast and sharpen fine features.
In order to help ensure that pattern features applied to a substrate are applied as intended (e.g. to ensure that critical dimension limits, requirements, or uniformities are met), it may be desirable to at least partially correct for aberrations in the lithographic apparatus. Aberrations may arise due to heating of one or more elements of a projection system of the lithographic apparatus due to the transmission or reflection of at least a portion of a radiation beam, and this heating may cause distortion or the like of those one or more elements. Alternatively and/or additionally, aberrations may arise for one or more other reasons, for example optical surfaces not performing in accordance with theory.